Radiation-induced inhibition of cell growth and proliferation will be studied, with particular emphasis on the depressed rate of DNA synthesis in the cell generation during which cells are irradiated, and on the deficiency in amount of DNA that is replicated in the next generation. HeLa S3 cells and 220 kV x-rays will be the main experimental materials. Treatment of irradiated cells with inhibitors of DNA synthesis, which results in enhanced cell killing, will be studied in further analysis of x-ray-induced potentially lethal damage.